Method for transporting liquified natural gas

ABSTRACT

A method of transporting natural gas by cooling and pressurizing retained natural gas to liquefy the retained natural gas within a fiber reinforced plastic pressure vessel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit from U.S. Provisional Application No.60/691,782, filed Jun. 20, 2005.

FIELD OF THE INVENTION

The present invention relates to a method of transporting natural gasand more particularly, the present invention relates to a method andsystem for transporting pressurized and liquefied natural gas.

BACKGROUND OF THE INVENTION

Low emissions and the high cost of oil have made natural gas the globalfossil fuel of choice. Currently, there are 6000 trillion cubic feet(TCF) of proven natural gas reserves in the world. Approximately half ofthose reserves are considered stranded; when it is not economical totransport by pipeline or ship-based liquefied natural gas (LNG). Bothpipelines and LNG have economical limits; pipelines in distance, LNG byproject and reserve size minimums.

Pipelines transport natural gas as a vapor, whereas LNG is transportedas a liquid. To liquefy natural gas at ambient pressure requirescryogenic refrigeration to −165° C. This is a costly and relativelycomplex process; however, due to the increased value of natural gas, theglobal demand for LNG has skyrocketed. Although this is the case,approximately TCF of proven reserves remain stranded.

To economically transport stranded and other natural gas reserves,various methods of Compressed Natural Gas (CNG) transportation methodshave been proposed and are in various stages of development. The mosttechnically feasible and cost effective method of CNG transportation isthrough the use of fiber reinforced plastic (FRP) pressure vessels.Unlike steel-based pressure vessels, FRP pressure vessels or bottles arelightweight, corrosion resistant, and have safe failure modes ifpunctured. The composite structure of FRP pressure vessels are resistantto temperatures as low as −80° C. or even lower; however, the port bossin the domes of FRP pressure bottles, used for connecting to manifoldpiping systems, are made of metal, and therefore, FRP bottles arelimited by the metallurgy used. Carbon steels loose strength and becomebrittle below temperatures near −40° C. Duplex, super duplex,precipitation hardened and titanium alloys in contrast maintain strengthand integrity in low temperatures; which therefore would allow thelow-temperature range of an FRP pressure vessel to be reached.

Lowering the temperature of natural gas while maintaining a constantpressure results in gas density increase. The concentrations of C1+hydrocarbons determine the thermodynamic characteristics of a particularmixture under varied temperature and pressure combinations. Higherdensity allows for higher volumes of gas that can be stored in the samespace, and therefore transported by ship, modal rail or roadway. Vaporpressure is somewhat proportionate to the proportions of larger carbonchain molecules in a gas mixture. A higher concentration of C2+ in amixture lowers the vapor pressure and therefore the inverse pressuretemperature combination that determines when a mixture begins toliquefy. The phase envelope for a particular natural gas mixture showsthe relative vapor/liquid proportion at any given pressure andtemperature combination. When fully liquefied, density within the phaseenvelope is maximized; however, a combination of gas and liquid may bemore practical for storing and or handling.

It has been found that the use of FRP pressure vessels to store naturalgas at low temperatures to partially or completely liquefy the said gasis effective and has wide commercial application. The use of FRPpressure vessels to store pressurized liquefied natural gas (PLNG)allows significantly large quantities of natural gas to be transportedby ship, tractor trailer, and modal container, or stored on land.Compared to compressed natural gas (CNG) stored at ambient temperature,the density and therefore net amount of natural gas is doubled bylowering the temperature by, as an example, forty to fifty degreesCelsius, at approximately half the pressure.

Using FRP pressure vessels to store PLNG is a safe, reliable,lightweight, corrosion resistant and cost effective way to transportnatural gas from source to market. It is also economically effective tostore natural gas on land for surge containment and storage.

Insulation of the FRP PLNG system will help keep the system cool andtherefore stabilize the liquid from boiling at sub-zero temperatures.

In view of the limitations in the art, it would be highly desirable tohave a method and a system for transporting greater quantities ofnatural gas.

The present invention satisfies this need.

SUMMARY OF THE INVENTION

One object of the present invention is to provide an improved method andsystem for transporting higher quantities of natural gas bypressurization and conventional thermal reduction to obtainliquefication.

A further object of one embodiment of the present invention is toprovide a method of transporting natural gas, comprising providing asource of natural gas, providing a fiber reinforced pressure vessel forretaining the natural gas, and cooling and pressurizing retained naturalgas to liquefy the retained gas within the fiber reinforced plasticpressure vessel.

A further object of the present invention is to provide a system fortransporting natural gas having fluid management apparatus and transportapparatus, the fluid management apparatus comprising a plurality offiber reinforced plastic pressure vessels for retaining the natural gas,fluid connection means interconnecting the pressure vessels, valve meansin fluid communication with the fluid connection means for admitting anddischarging the gas exteriorly of the vessels or between the vessels,support means for supporting the plurality of fiber reinforced pressurevessels, the fluid transport apparatus, comprising a vehicle forreceiving the fluid management apparatus, cooling means for cooling thenatural gas, and pressurizing means for pressurizing the natural gas,whereby pressurized and liquefied natural gas is transportable with thevehicle.

Yet another object of one embodiment of the present invention is toprovide a system for transporting natural gas having fluid managementapparatus and transport apparatus, the fluid management apparatuscomprising a plurality of fiber reinforced plastic pressure vessels forretaining the natural gas, fluid connection means interconnecting thepressure vessels, valve means in fluid communication with the fluidconnection means for admitting and discharging the gas exteriorly of thevessels or between the vessels, support means for supporting theplurality of fiber reinforced plastic pressure vessels, the fluidtransport apparatus, comprising a vehicle for receiving said fluidmanagement apparatus, cooling means for cooling the natural gas, andpressurizing means for pressurizing the natural gas, whereby pressurizedand liquefied natural gas is transportable with the vehicle.

Having thus generally described the invention reference will now be madeto the accompanying drawings illustrating preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical illustration of the phase envelopes for naturalgas;

FIG. 2 is an enlarged view of the manifold system and FRP vessels asconnected according to one embodiment;

FIG. 3 is a top view of the ship hold with the FRP pressure vessels heldin position by a cassette modular framing system;

FIG. 4 is a side view of FIG. 3;

FIG. 5 is a perspective view of a cassette support framing systemaccording to one embodiment of the present invention;

FIG. 6 is a view of the cassette with an FRP in situ together with themanifold system;

FIG. 7 is a side view of a group of individual stacked cassettes withmodules in position;

FIG. 8 is a view of another vehicle for retaining the FRP vessels; and

FIG. 9 is a view of a land based system.

Similar numerals denote similar elements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For modes of PLNG transportation and storage including a ship, thecombination of low temperature and pressure to increase density near orto the point of liquefaction can be further optimized by increasing theC2+ concentration of the gas mixture. It is known that increasedconcentrations of C2+ in a gas mixture, lowers the vapor pressure of theentire mixture. Thus, higher concentrations of C2+ in the gas mixturewill allow for larger net volumes of natural gas to be stored andtransported comparatively. This is generally depicted in the phasediagram of FIG. 1.

Using a vertically oriented FRP PLNG gas containment system, natural gasmay be discharged from the containment system as a vapor or a liquid.Vapor may be discharged through the upper manifold piping system. Liquidnatural gas may be discharged through the lower manifold system. Tocounteract Joule-Thompson effects during de-pressurization and maintainminimum/maximum temperatures in the system, some heat may have to beapplied. In one possibility, the heat could be applied directly to oneor more manifolds.

The thermodynamic characteristics of a natural gas/liquids mixture aredetermined by the concentrations of C2 and C3+ in the mixture. Thehigher the concentration of C2+, the lower the vapor pressure of themixture. Therefore, by adding or maintaining a significant C2 and C3+concentration, a relatively low vapor pressure may be obtained. A lowervapor pressure will allow for the gas being injected into a FRP PLNGstorage system to liquefy with less pressure or different temperature,than with a higher vapor pressure.

By making use of the thermodynamic characteristics, control of the boilrate during discharge permits significant proportions of C2 and C3+hydrocarbons to remain as a liquid in the FRP PLNG system. This obviatesthe requirement of having to remove C2 and C3+ hydrocarbons beforeinjecting the gas into a pipeline distribution network. Most pipelinedistribution systems have a restriction on the thermal content of gasentering into a pipeline system. In North America, the limit isgenerally 1050 Btu's (British thermal units) per scf (standard cubicfeet) of gas.

As the pressure in the FRP PLNG system is reduced at assumed constanttemperature, the vapor pressure of the liquid/gas mixture is increased.This will induce more gas to boil. Controlling the rate of pressure dropand temperature change in the storage system will therefore control theboil rate of liquid gas. When the boil rate is constricted, the tendencyis for the lighter hydrocarbons to boil first. C2, but moreover, C3+hydrocarbons tend to stay as a liquid. Thus, as the liquid/vaporinterface lowers toward the bottom of the FRP bottles at a constrainedrate, the concentration of C2 and C3+ molecules increases. Thepropensity is for the heaviest molecules to collect over repeated cyclesas they are less likely to vaporize at a constrained rate of boil. Theheavier hydrocarbon concentration change during discharge of the cargowill also change the vapor pressure of the liquid gas mixture. Thegreater the concentration of C2+ hydrocarbons, the lower the vaporpressure of the changing mixture.

Maintaining a low temperature in the FRP PLNG containment system duringdischarge, a high concentration of C2 and C3+ will remain as a liquid atthe bottom part of the system. This C2 and C3+ mixture can then bereturned to the source of natural gas and reused for the next shipmentwithout processing the gas externally of the containment storage systemto remove C2 and C3+.

As the concentration of C2 and C3+ builds over time, some C2 and C3+ maybe used for power generation on board the ship. There will be aneconomical crossover point where additional C2 and C3+ hydrocarbons nolonger increase the net amount of cargo transported on a PLNG carrier ormodal system. Any C2 and C3+ over this amount would not be economicallyadvantageous. The most cost effective system will be at the crossoverpoint.

Alternatively, PLNG may be discharged through the lower manifold andre-gasified on deck for offloading. It may even be offloaded as a liquidif desired for direct injection into a land-based storage system. Ifthis alternative is chosen, then some C2 and C3+ liquids used to achieveincreased density could be extracted separately and restored for thereturn journey.

C2 and C3+ concentrations in a land based FRP PLNG storage system wouldhave the same or similar density/capacity increase effect within anequal space.

This method of PLNG storage would also be cost effective to transportethane (C2) as a commodity of its own. Ethane is the feedstock for thepetrochemical industry. It therefore has a significant commodity value.Ethane is currently only transported by pipeline. The feedstock to thepetrochemical industry is therefore limited to sources obtainable bypipeline. PLNG offers another transportation mode of much largerdistances than feasible via pipeline transport.

To overcome thermal input to the system during compression and loading,the residual C2 and C3+ hydrocarbons can be chilled to the minimumtemperature allowed at a specified pressure. During the return journey,the residual natural gas liquids and captured C4 and C5+ hydrocarbons,may be super-chilled without danger of rapid depressurization causing atemperature drop. The pressure drop would be negligible. Therefore, whenmixed with new and possibly hot gas coming into the system, thetemperature will equalize and remain as low as possible and, as requiredto achieve the affect desired. If incoming gas into the system isthrough the lower manifolds, the incoming gas would have to percolatethrough the heavy hydrocarbon residual. This would help to mix the heavyhydrocarbons stored in the bottoms of the systems to mix with theincoming gas.

With reference to FIGS. 2 through 4 shown as a vehicle, shown in theexample as a ship 10 with the FRP pressure vessels generally denoted bynumeral 12. The vessels each have an upper metal alloy port boss 14 anda lower metal port boss 16 which may be composed of the metals notedherein previously (duplex, super duplex, precipitation hardened) andother suitable stainless steels of similar grade. The individual portbosses are connected by upper and lower piping manifolds 18, 20,respectively. The piping manifolds 18 and 20 will be selected of similarmaterials as the port bosses and will have the feature of being capableof withstanding low or ultra low temperatures.

The FRP vessels 12 may be held in modular cassette frames, denoted inFIG. 5 by numeral 22. The cassette frames 22 can be stacked and nestedin the hold of a ship as is indicated in FIGS. 3 and 4. The frame isdesigned to isolate the vessels including the piping manifolds from shipmovement and vibration. It is also useful to facilitate full visualinspection of the fiber reinforced plastic pressure vessels while inservice. The cassette is composed of a frame with a bottom grid 24 whichis for the purpose of supporting the vessels (the vessel is not shown inFIG. 4). The frame has three sides 26, 28 and 30 and an open top. Thelack of a top section is to facilitate ease of installation for thevessels into frame 22 and also is useful from a mass point of view; theabsence of a top and one or more sides reduces the overall mass.

Once installed in the hold of a ship as shown in FIG. 4 the adjacentcassette frames can be bolted together and include a bushing 32 (seeFIG. 5) to absorb hydrodynamic movement during traveling. Where thecassettes 22 are stacked in a vertical manner, it will be evident thatthe bottom grid 24 of the upper cassette provides for lateral bracing ofthe lower cassette frame as is clear from FIG. 4.

Each cassette frame 22 is equipped with upper and lower piping manifolds18 and 20 respectively, to connect the top and bottom 14 and 16 portbosses of vertical vessels 12. The bottom manifold 20 is secured to thegrid 24 of the cassette 22. The upper manifold 18 is also securedhowever, it is guided by guides 34 to allow for elongation of thepressure vessels during pressurization. This is illustrated in FIG. 6.The connection of the vessels 12 to pipe it through the piping manifolds18 and 20 may be directly welded or via high pressure flange connections(not shown) which are integral with the port bosses 14, 16 of thevessels 12.

To create a stack or cluster of cassette modules 22, the upper manifold18 of a lower cassette may be connected to the lower manifold 20 of theupper cassette. The lowermost and uppermost manifolds would then beconnected to the respective piping that would lead to the firstisolation valves located on the deck of the ship 10. The uppermost andlowermost manifolds denoted by numerals 36 and 38 in FIG. 7 would beconnected to isolation valves located in the deck of ship 10, whichvalves are denoted by numerals 40 and 42, the latter illustrated in FIG.4.

As an option, the manifold piping may be insulated with suitableinsulation denoted by numeral 44 in FIG. 2 or the entire cassette systemmay be composed of insulated frames. As a further possibility, theinside of the ship's hold may be insulated.

On the main deck of the ship 10, there is included refrigeration andcompression equipment, globally denoted by numeral 46 in FIG. 4.

Turning to FIG. 8, shown is a further embodiment of the invention wherethe individual cassettes 22 have been installed on a trailer 50.Suitable pressurization and compression equipment may be included onboard the trailer (not shown) or simply extraneous of the trailer 50.

FIG. 9 schematically illustrates a land based system 52, where the samecomponents are incorporated from FIG. 8 with exception that the trailer50 (FIG. 8) is deleted and replaced by frame 52.

Although embodiments of the invention have been described above, it islimited thereto and it will be apparent to those skilled in the art thatnumerous modifications form part of the present invention insofar asthey do not depart from the spirit, nature and scope of the claimed anddescribed invention.

1. A method of transporting natural gas, comprising: providing a sourceof natural gas; providing a fiber reinforced plastic pressure vessel forretaining said natural gas; and cooling and pressurizing retainednatural gas to liquefy said retained gas within said fiber reinforcedplastic pressure vessel.
 2. The method as set forth in claim 1, furtherincluding adjusting the concentration of at least one of C2 and C3+present in said natural gas during storage of said natural gas todecrease the vapor pressure thereof.
 3. The method as set forth in claim1, further including maintaining said C2 and said C3+ in a liquid stateduring storage for recycling to said source of natural gas.
 4. Themethod as set forth in claim 1, further including maintaining said C2and said C3+ in a liquid state in said source of said pressurized andliquefied natural gas during discharge of said fiber reinforced plasticpressure vessel.
 5. The method as set forth in claim 4, wherein saidnatural gas is discharged/de-pressurized in a controlled manner forcontrolling the boil rate of pressurized and liquefied natural gas toincrease the concentration of C2 and C3+ remaining in said vessel duringdischarge/de-pressurization.
 6. The method as set forth in claim 5,wherein said C2 and said C3+ remaining in said fiber reinforced pressurevessel subsequent to discharge of said natural gas is further cooledduring return journey to the source of natural gas.
 7. The method as setforth in claim 5, wherein chilled C2 and said C3 are retained in saidvessel and mixed with natural gas during reloading of said natural gasinto said pressure vessel to lower the temperature of reloaded naturalgas and the resulting mixture.
 8. The method as set forth in claim 7,wherein retained and super-chilled C2 and said C3+ collectively lowerthe vapor pressure of the mixture of C2 and said C3+ and natural gas. 9.The method as set forth in claim 1, wherein in alternation, pressurizedand liquefied natural gas is discharged from said vessel through a lowermost manifold connected thereto.
 10. Use of a fiber reinforced plasticpressure vessel for retaining pressurized and liquefied natural gas. 11.The use as set forth in claim 10, wherein said vessel includes valvemeans for admitting and discharging said gas, said means composed of asteel selected from duplex, super duplex and/or precipitation hardenedstainless steel.
 12. The use as set forth in claim 10, wherein saidvessel is composed of a material selected from the group consisting ofglass, carbon, and aramid filament fiber.
 13. The use as set forth inclaim 10, wherein said vessel has an operating temperature below atleast −50 C.
 14. A system for transporting natural gas having fluidmanagement apparatus and transport apparatus, said fluid managementapparatus comprising: a plurality of fiber reinforced plastic pressurevessels for retaining said natural gas; fluid connection meansinterconnecting said pressure vessels; valve means in fluidcommunication with said fluid connection means for admitting anddischarging said gas exteriorly of said vessels or between said vessels;support means for supporting said plurality of fiber reinforced plasticpressure vessels; cooling means for cooling said natural gas; andpressurizing means for pressurizing said natural gas.
 15. The system asset forth in claim 14, wherein said support means comprises a cassetteframe.
 16. The system as set forth in claim 14, wherein said fluidconnection means comprises a manifold network interconnecting individualvessels.
 17. The system as set forth in claim 14, wherein each saidvessel includes at least one set of first and second opposed valves. 18.The system as set forth in claim 14, wherein said vehicle is selectedfrom the group consisting of a marine vessel, automobile and train. 19.A system for land based storage of natural gas, comprising: a pluralityof fiber reinforced plastic pressure vessels for retaining said naturalgas; fluid connection means interconnecting said pressure vessels; valvemeans in fluid communication with said fluid connection means foradmitting and discharging said gas exteriorly of said vessels or betweensaid vessels; support means for supporting said plurality of fiberreinforced plastic pressure vessels; cooling means for cooling saidnatural gas; and pressurizing means for pressurizing said natural gas.